In nuclear reactors, in particular in pressurized water nuclear reactors, the core of the reactor consisting of a set of prismatic assemblies, generally of square cross-section and arranged vertically, is traversed by a cooling liquid such as water under pressure, in the direction longitudinal to the assemblies, for cooling them and for the transfer of the heat of the core to the steam generators.
The fuel assemblies generally consist of a set of tubes enclosing the fissile matter, which are joined together in the form of a bundle thanks to retainer grids which are transverse with respect to the tubes forming the assembly.
The whole of the core is arranged in an enveloping skirt which is generally of cylindrical shape like the tank which contains the core and its auxiliary equipments immersed in the cooling liquid. This envelope must be separated from the assemblies of the core in order to diminish the injurious effects of irradiation upon the material of which it is formed. On the other hand, the set of assemblies forming the core exhibits a cross-section of irregular shape, including steps which are inscribed within the area of the enveloping skirt, with the result that there exists between the peripheral assemblies and the enveloping skirt an annular zone which is empty of fuel.
In order to preserve the initial geometry of the core and prevent the coolant from flowing freely in this zone and channel it over the core proper, the practice is known of employing a partitioning unit which exactly follows the contour of the core.
This partitioning unit is brought to a temperature close to that of the core with which it is in contact, whereas the cylindrical envelope of the core is at a substantially lower temperature.
That is, the cooling fluid, having given up a portion of its calories to the fluid which is to be evaporated in the steam generators and having been reintroduced into the reactor tank, flows in contact with the outer surface of the envelope of the core before coming into contact with the fuel assemblies forming the core. Hence this recycled fluid is at a distinctly lower temperature than the temperature of the core. The devices for partitioning off employed to date consist of plates in the longitudinal direction, the length of which is substantially equal to the height of the core and which are connected rigidly to the envelope of the core by way of transverse reinforcing plates called blenders, the latter being arranged at regular intervals in the direction longitudinal to the plates for partitioning off, in the space surrounding the core.
It is in fact necessary to connect the plates for partitioning off and the cylindrical envelope rigidly by elements of great strength, so as to enable the core to resist stresses such as those brought into play, for example, during earthquakes.
Such a structure must also be constructed with high dimensional accuracy in order to be perfectly adjusted to the dimensions and the shape of the unit forming the core of the reactor.
Hence thermal stresses caused by the difference in temperature between the envelope of the core and the plates for partitioning off arise in the structure for partitioning off, which is connected rigidly to the envelope of the core.
In addition, for the achievement of the structure for partitioning off, it is necessary to have available plates of very large dimensions, perfectly machined, and these plates are tricky to produce and of high cost.